The conventional hoist and traction machine, as disclosed in, for example, the Japanese Utility Model Laid-Open Gazette No. Sho 63-142389, has been used in a lever type hoist and traction machine, a manual hoist and traction machine or a motor-driven hoist and traction machine. The mechanical brake comprises a driven member rotatable together with a driving shaft,a driving member forwardly screwable therewith, a braking ratchet wheel engageable with a braking pawl, and lining plates disposed to sandwich therebetween the braking ratchet wheel, the ratchet wheel and lining plates being interposed between the driven member and the driving member. For example, in the manual hoist and traction machine, when a load is hoisted or haulded, the driving member is driven by a lever, a hand wheel or the like, so that the driving member, when driven, screws toward the driven member to bring the braking surface of the driving member into press-contact with one lining plate and further screws forward to bring one lining plate and the other into press-contact with the braking surfaces of the driven member and braking ratchet wheel respectively, thereby transmitting a braking force from the driven member to the driving shaft. Also, when the driving member is subjected to no driving input, the braking ratchet wheel engages with the braking pawl so as to be stationary and to restrain reverse rotation and the mechanical brake operates to brake the reverse rotation of the driving shaft caused by a load, thereby enabling the load to be held in the state of being hoisted or haulded.
In a case where such braking slate is released to lower the load or release traction thereof, the driving member is reversely rotated to screw brackwardly with respect to the lining plate, thereby releasing the braking force.
In a case where the load is lowered or the traction thereof is released during the operation of the mechanical brake, the driving member is reversely rotated to screw backwardly, thereby releasing the braking action of the mechanical brake as the above-mentioned, in which a predetermined releasing force corresponding to the load, but changeable therewith at the initial stage, is required. In other words, when the load acts on a load sheave in association with the driving shaft, a force by the load, which is intended to reversely rotate the load sheave, is transmitted to the driving member and allows it to screw toward the lining plate, the force being converted into a tightening force for the lining plate and becomes larger as the load is larger. Therefore,for lowering the load or the releasing the traction, the driving member braked by the tightening force screws backwardly to release the braking action and in the initial stage of screwing backwardly the driving member to release its braking action, a force overcoming the tightening force must be input into the lever or hand wheel for screwing backwardly the driving member. In other words, when the load is lowered by the lever or the like, a relatively smaller force than the tightening force set corresponding to the load, after the load lowering has started, can be used to lower the load, but in the initial stage of load-lowering operation a large releasing force corresponding to the tightening force is required.
In the state where cargos on a load carrying platform of a truck are tightened by traction operation of the machine, when the traction and cargo-tightening are released, in the initial stage, similarly, a large releasing force caused by backward screw of the driving member and corresponding to the tightening force of the mechanical brake is required.
However, in the conventional mechanical brake, the braking surfaces of the driving member, driven member and braking ratchet wheel, all are formed of the surface of raw material so as not to control the coefficient of friction, whereby the release of braking force at the initial stage of load lowering operation varies so that the releasing force larger than the tightening force corresponding to the load may be required. And, depending on the material of lining plates, the above-mentioned members may be attacked by the lining plate to vary in the surface condition, or an external condition, such as rusting, may be added to vary the surface condition, whereby the coefficient of friction increases so as to increase the required releasing force and occasionally the braking action cannot be released.